add back-end implementation for SSO JIT provisioning

[roperzh]

Related to #7053, this uses the SSO config added in https://github.com/fleetdm/fleet/pull/7140 to enable JIT provisioning for premium instances.

A few notes:

1. Documentation and changelog will be added in the UI PR
2. I changed the integration tests for SSO to use an actual SSO server instead of a hardcoded response, and added tests for the whole login flow (instead of just the config endpoint.) If we think this is not a good idea I can revert
3. I couldn't find an standard for users' full names in SSO callback responses, so I collected the most commonly used attribute names and we check for any of them to be present. Documentation for this will also be included in the UI PR.

https://user-images.githubusercontent.com/4419992/184360072-115fe21c-6fed-4150-bb99-4991c3143e53.mov

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• [x] Ensured that input data is properly validated, SQL injection is prevented (using placeholders for values in statements)
• [x] Added/updated tests
• [x] Manual QA for all new/changed functionality

[codecov-commenter]

Codecov Report

Merging #7182 (8441818) into main (acf6018) will increase coverage by 0.13%. The diff coverage is 79.38%.

@@            Coverage Diff             @@
##             main    #7182      +/-   ##
==========================================
+ Coverage   60.12%   60.26%   +0.13%     
==========================================
  Files         408      408              
  Lines       38553    38635      +82     
==========================================
+ Hits        23180    23283     +103     
+ Misses      13116    13085      -31     
- Partials     2257     2267      +10     

Impacted Files	Coverage Δ
pkg/fleethttp/fleethttp.go	82.00% <0.00%> (-13.35%)	:arrow_down:
server/fleet/activities.go	0.00% <ø> (ø)
server/fleet/sessions.go	0.00% <ø> (ø)
server/service/metrics_sessions.go	0.00% <0.00%> (ø)
server/service/service_users.go	34.48% <50.00%> (ø)
server/service/sessions.go	73.43% <68.00%> (+6.92%)	:arrow_up:
server/service/testing_client.go	96.27% <94.11%> (-1.00%)	:arrow_down:
server/service/users.go	68.52% <100.00%> (+0.49%)	:arrow_up:
server/sso/authorization_response.go	76.00% <100.00%> (+10.14%)	:arrow_up:
... and 2 more

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[roperzh]

Sorry for the late change! I pushed one last commit to validate emails 590fb44